# How does a hygrometer's accuracy change?



## StogieNinja (Jul 29, 2009)

Tried a brief search, but couldn't find an answer. I have no idea how a hygrometer actually works, other than that it measures ambient humidity. So I have two questions:

Have any of you bought hygrometers which over time varied in their accuracy? For example, if I buy a cheapo hygro, I can put it in a ziplock with a Boveda and figure out how far off the hygro is now... but how long will that difference last? And secondly, can it vary based on rH?

In otherwords...

1. If when calibrating, I discover the hygrometer's accuracy is off by a difference of 4%, will the hygro _always _be off by 4%, or is it likely that it will slowly become less accurate over time?

and

2. A scale that has a tare set will always be "off" by the same exact amount, whether weighing one lb or ten lbs. Does a hygrometer work the same way? Or does it work more like a pendulum, where if it's off a little at the low end, it'll be off a lot more at the far end? Can a hygrometer's accuracy vary based on how high or low the rH is?


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## Habanolover (Feb 22, 2006)

Derek, all the hygrometers I have seen will keep within the same range regardless of whether the RH is higher or lower. In other words, if you are off 4% at 65% humidity it will also be off 4% at 85%. I have also found that the cheaper models most use in their humidors/coolers, etc. will become less accurate over time. Of course this is all dependent on whether the batteries are changed regularly.

Take my word with a grain of salt since I don't use hygrometers and haven't in about 5 years.


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## Johnpaul (Aug 15, 2011)

Hygrometers (both analog and digital) loose their accuracy over time due to them having to operate in open air and so they get contaminated with whatever is in the air (dust, etc.) That being said I don't think contaminates affect the accuracy curve as much as they just cause some drift (slowly over time) All hygrometers have a accuracy curve and I believe that they are most accurate within 20% of where they are calibrated. In other words if your meter can read 20%-100% and you calibrate at 75% then the readings will be most accurate at ~66%-84%. With cigars we don't really need to be all that accurate however and any good quality hygro should be able to hold ~3% accuracy within 30% of the calibrating point and that should cover anyone's needs as far as cigar storage goes.

Now with that being said their are different kinds of sensors if different meters. A good quality hair hygro is much more accurate than your standard crap that comes with a $20 humi. The same applies to digital. Their are different kinds of humidity sensors and it is my belief that when you buy the cheap generic units you get very low quality sensors with less accurate readings.


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## Bondo 440 (Jul 24, 2012)

The cheap analogue ones I have four of them. So far the failures are pretty much the same. When you do your salt test, and see 70% RH for example, you will adjust the main screw in the back, to set your hygrometer to 75% . We all know that, Now here's the trick.---

When you turn the screw, you aren't adjusting anything. No you're not. The dial and the entire assembly is simply being rotated. The pointer is not moving, technically. The entire spring, housing, and the attached dial are simply moving as one assembly around the faceplate. Make any sense? 
So basically, when you "calibrate" your analog hygrometer from 70 to 75 % , you are, in reality, not doing anything except rotating the dial face to put the number 75 under where the pointer is. The pointer isn't moving really. The whole assembly is being turned so the pointer is under the 75 on the face plate.
_
The adjustment "screw" that is not a screw at all. 
It's really just a slot to reposition the dial assembly under the face plate._



















Take one apart and see for yourself. I had one that if you touched the thing it's dial would move. I opened it all apart and the problem is that on these devices, the face plate actually "floats" inside the assembly. I tacked it down. It never failed me again so far.

One of the most embarrassing design flaws I have seen, deals with the front facing analog dials in the humidors. If you remove the hygrometer, you will see a wide rubber belt goes around the unit. You will also notice the belt covers where all the holes are to read the RH. So basically, in the front facing applications, you block them all with the mounting band.

_Here is how the hygrometer reads your RH. Let's block them. _










So where does it get it's air to interpret it's reading from? The only hole left. The single adjustment screw hole on the back. Since the unit is in the "wall" of the humidor, only the RH that makes it into the hole, is the actual RH of within the storage chamber. Since the front glass lens is entirely exposed to the outside environment, I am not surprised that a perfectly dependable and tested analog hygrometer will read -5% lower RH in this position, than if you remove the belt and relocate the same unit totally inside the humidor on a tray.

Hopefully this helps. So how does a hygrometer's accuracy change? The simple assembly that does the actual reading, is about as complicated in it's working design, as a thermometer. It's likely not changing at all. It is travel on that main assembly or movement of the front facade that gives you the erroneous readings. And of course, mounting inside a front panel. Go digital when you get a chance, but the analog will work once you understand it. At least until you can do better.


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## KaChong (Jan 1, 2012)

A post I made awhile ago on a similar thread:



KaChong said:


> When I took apart my hygro for a coolidor build, I rewired the hygro sensor so I could read the LCD display outside of the coolidor and have the sensor inside. I didn't want to disturb the humidity level to check the humidity. The sensor in the rectangular Xikar sensor looks like this:
> 
> 
> 
> ...


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## Bondo 440 (Jul 24, 2012)

Looks good , covers ( "uncovers" ) the electronic hygrometer well. I agree the hygrometer node of the assembly seems to be a glorified variable resistor. All the rest of the work is done by the IC. Thanks for putting the electronic sensors photo next to my post. Very Cool.


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